The present invention relates to controlling the timing of sequential emissions of a liquid from a spray head, or the like; more specifically, the invention relates to automated control of the time interval between emissions of a liquid, e.g., an adhesive, ink, paint, etc., which is directed from a solenoid-actuated nozzle to a succession of items which are moving in spaced relation to one another and to the nozzle transversely to the direction of travel of the liquid.
For purposes of the present discussion, the liquid applied from the spray head will be assumed to be an adhesive. Liquid adhesives are commonly applied in the form of a spray to predetermined areas of items which are moved past a spray head in a direction perpendicular (or otherwise non-parallel) to the direction in which the adhesive spray is emitted. In automated packaging systems, for example, adhesive is applied by a spray head to designated areas of boxes, cartons, or other containers, which are then adhered to an opposing surface to effect closure of the container. There are, of course, many other industrial applications of the automated application of liquid adhesive in spray form to the areas requiring adhesion to one or more other items or areas.
Typical automated adhesive spray installations include a spray head, including nozzle and solenoid-actuated valve, a photodetector and a control unit. Liquid adhesive is supplied to the head under pressure and is emitted in a predetermined pattern through the nozzle when the valve is open. The photodetector is positioned to generate a signal in response to the leading edge, or other portion of the item to which adhesive is to be applied, passing a predetermined point as it is moved, e.g., on a conveyor belt, in a path through the adhesive application area. The photodetector signal is applied to the control unit which actuates the solenoid.
In order to ensure that the adhesive is applied to the desired target area, and only to this area, precise timing of opening and closing of the valve is required, also taking into account the distance of travel and velocity of the adhesive. Although the distance is a known and essentially non-variable parameter, velocity is influenced by such factors as delivery pressure, temperature and viscosity of the liquid adhesive, any of which may be subject to fluctuations. In addition to variations in adhesive velocity, electrical and mechanical response times of the solenoid coil and associated valve will have an effect on the area of application of the adhesive. Typical packaging applications require timing accuracy within 5 milliseconds, and tolerances of plus or minus one millisecond of the ideal timing are not uncommon.
It is the current practice to measure the factors influencing the proper timing of valve opening and closing, and apply a fixed compensation factor via the control unit. That is, the time-off-light of the adhesive from the nozzle to the target, the delay due to inductance of the solenoid coil and the mechanical response time of the valve are lumped together and a single time value, typically 8 to 20 milliseconds, is applied as what is termed a head compensation or "head comp" factor.
In typical installations, head comp settings and adjustments are made by a technician observing the area of adhesive application with a strobe light when the production line is at full speed operation. Although such head comp settings may be reasonably accurate at the moment they are made, besides being subject to human error they may be invalid in a relatively short time due to fluctuations in one or more of the aforementioned parameters. This leads to errors in positioning of the area of adhesive application which are amplified as the speed of production increases.
Accordingly, it is a principal object of the present invention to provide a novel and improved system for controlling the application of liquid from a spray head to a moving item, ensuring accurate application of adhesive to a desired target area.
Another object is to provide an automated method of adjusting the head comp setting of a liqiud spray head.
A further object is to provide an automated system for delay-compensated control of a liquid spray installation with the capability of monitoring ejection of adhesive from the spray head.
Other objects will in part be obvious and will in part appear hereinafter.